1. Field of the Invention
The invention relates to a process for the preheating and multistage deaeration of make-up water in a power generation plant by means of steam. It also relates to an equipment arrangement for carrying out the process.
2. Discussion of Background
The consumption of spent deaerated demineralized water in combined and industrial power station plants is very large. This necessarily leads to the treatment of considerable quantities of make-up water for the purpose of replacing the losses. In this connection, special condenser configurations with mixing preheaters/deaerators are known, which are capable of preheating and deaerating quantities of make-up water of up to 70% of the quantity of the steam released. Normally, not more than 3 to 5% of make-up water, relative to the quantity of steam released, is injected in classic condensing power station plants directly into a condenser. However, massive injection of water adversely affects the condenser pressure, since the condenser bundles are supplied with external water, that is to say, water which does not come from the condensation. The direct injection of make-up water in the condenser would, because of the large quantities mentioned, lead to flooding of the tube bundles. A loss of vacuum would then be inevitable, which represents a considerable impairment of the condenser operation.
Economical preheating and deaeration of large quantities of make-up water is nowadays carried out with steam of the lowest energy level, the overall efficiency of the power station process being adversely affected only to a minimum extent. In order to achieve this object according to the conventional practice, packed columns are installed above the condenser, wherein the expulsion of the gases dissolved in the make-up water takes place with the use of the turbine exit steam as a stripping agent. In this case, the required condenser pressure is maintained with the aid of an additional suction device.
When the make-up water, in most cases trickling in, enters a packed column operated in countercurrent, this make-up water is as a rule subcooled by 10.degree. C. to 18.degree. C. relative to the stripping steam. For ideal deaeration in a packed column, however, approximately thermal equilibrium between the liquid phase and the gas phase is necessary. Because of the demonstrated subcooling, the waste steam must thus initially accomplish the thermal saturation of the make-up water. If the preheating is to take place in a packed column in the same way as the deaeration, the column cross-section of such a packing must be designed with excessive dimensions e.g., be oversized because of the possible flooding risk. The design of a packed column for the such loading is, however, associated with high costs. The steam which, during the preheating of the downward-flowing water, is driven ineffectively through such a packed column, represents an inevitable loss from the water-steam circulation, since efficient deaeration, as mentioned, takes place only after the temperature of the make-up water has approached that of the stripping steam to within less than 1 K. Furthermore, the installation of a further suction device in a water-steam circulation is necessary, if the packed column is also used for the heating of the make-up water.
Effective deaeration is characterized by a deaeration range from 10,000 ppb (parts per billion) of O.sub.2, which is the state of saturation of the water with atmospheric air at room temperature, down to single-digit ppb values such as about 5 ppb.